High purity hot pressed boron nitride

ABSTRACT

High purity boron nitride articles of manufacture having a high density and improved properties. The improved boron nitride articles have a unique acicular crystal structure, an oxygen content of less than 0.5 percent by weight, a density of at least 1.9 grams/cc., excellent hot strength, low and substantially isotropic coefficients of thermal expansion with no irreversible thermal expansion, excellent thermal shock resistance, moisture insensitivity and improved dielectric properties. These articles are produced by treating conventional hot pressed boron nitride articles with a suitable solvent to lower their boron oxide (B2O3) content, and then sintering the treated material in an inert atmosphere at a temperature of from 1,600* C. to 2,100* C. in the absence of pressure or mechanical restraint.

United States Patent Mandorf, Jr. et al.

[ 1 May2,1972

[541 HIGH PURITY HOT PRESSED BORON NITRIDE [72] Inventors: VictorMandorf, Jr., Olmsted Falls; Lionel C. Montgomery, North Olmsted, bothof [21] App1.No.: 822,214

[52] U.S.CI ..23/l91 [51] .C0lb 21/06 [58] Field ofSearch ..23/l91 [56]References Cited UNITED STATES PATENTS 2,808,314 10/1957 Taylor ..23/1913,058,809 10/1962 Taylor i i ..23/191 3,241,918 3/1966 Lenihan et a1.......23/l91 3,420,629 1/1969 Lubatti et a1. ..23/19I FOREIGN PATENTS ORAPPLICATIONS 483,201 4/1938 England ..23/191 1,073,936 6/1967 England..23/l91 OTHER PUBLICATIONS Ingles et a1., The Preparation & Propertiesof Boron Nitride, from Special Ceramics, pp. 144- 167, Academic Press1960).

Primary Examiner-Oscar R. Vertiz Assistant Examiner-Charles B. RodmanAttorneyPaul A. Rose, Robert C. Cummings and John S. Piscitello [5 7]ABSTRACT High purity boron nitride articles of manufacture having a highdensity and improved properties. The improved boron nitride articleshave a unique acicular crystal structure, an oxygen content of less than0.5 percent by weight, a density of at least 1.9 grams/co, excellent hotstrength, low and substantially isotropic coefficients of thermalexpansion with no irreversible thermal expansion, excellent thermalshock resistance, moisture insensitivity and improved dielectricproperties. These articles are produced by treating conventional hotpressed boron nitride articles with a suitable solvent to lower theirboron oxide (B 0 content, and then sintering the treated material in aninert atmosphere at a temperature of from 1,600 C. to 2,100 C. in theabsence of pressure or mechanical restraint,

12 Claims, 2 Drawing Figures PATEHTEB MAY 2 I972 0R5 JR. LIONEL C.MONroom/w INVENTORS VIC TOR MA ND ATTORNEY HIGH PURITY HOT PRESSED BORONNITRIDE BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to high purity boron nitride articles. Moreparticularly this invention relates to high purity hot pressed boronnitride articles of manufacture having a high density and improvedproperties.

2. Description of the Prior Art Boron nitride articles are generallyfabricated by pressmolding and sintering techniques. The techniquesemployed are broadly classified as:

a. The hot pressing method: wherein sintering (at 1,700C. to 2,200C.) iscarried out simultaneously with the pressure molding process.

b. The cold pressing method: wherein press molding is first carried out,and then sintering is carried out.

A modification of the cold pressing method is disclosed in UnitedKingdom Pat. No. 1,073,936 whereby cold pressed boron nitride powder issintered by heating in a mold without the application of pressure butwhile free expansion of the boron nitride due to heating is restrictedby the mold.

Boron nitride articles produced by the hot pressing method offabrication have a high boron oxide (B content. While this material actsas a binder during hot-pressing and allows such articles to be hotpressed from boron nitride,-it also causes a weakening of the propertiesof the hot pressed articles. Thus, because of the presence of thismaterial, hot pressed boron nitride articles are weak at hightemperatures (flexural strengths of about 3,000 psi. or lower at 1,000"C.), exhibit a permanent expansion upon heating to l,800 C. and coolingto room temperature of close to 1 percent or more, and pick upsufficient moisture (which chemically combines with the boron oxidepresent to produce boric acid) under normal humidity conditions to causecracking if the boron nitride is exposed to a rapid rise in temperature.These properties have restricted the use of boron nitride produced inthis manner in many high temperature applications where a strongmaterial with reversible thermal expansion and high thermal shockresistance is required.

Boron nitride articles produced by the cold pressing method are of lowdensity (about 1.5 grams/cc. to 1.6 grams/cc.) and low flexural strength(about 3000 psi. from room temperature to 1,000 C.). DUring thesintering stage the cold pressed material undergoes a thermal expansionwhich remains even after the product is cooled to room temperature. Thispermanent expansion is caused by the presence of boron oxide (B 0 and isof the order of 4.5 to 5 percent when sintering temperatures of 1,500C.to 2,100C. are employed. Further, because of the expulsion of boronoxide (B 0 during sintering, it has not been possible to produce coldpressed articles greater than A inch to 2% inch in thickness (smallestdimension) without cracking them.

While United Kingdom Pat. No. 1,073,936 attempts to overcome theirreversible thermal expansion which cold pressed boron nitrideundergoes when it is heated to sintering temperatures (and therebyincrease its density) by conducting this operation while restricting thefree expansion of the boron nitride in a mold, this method has only beensuccessful in raising the bulk specific gravity of such materials from 6to 9 per cent to as high as 1.705. F urthermore, the pressure exerted bythe escape of boron oxide (B 0 during sintering is usually sufficientlygreat to crack the mold as well as the boron nitride piece itself. Inaddition, this method does not produce any significant increase inflexural strength over that of conventional cold pressed boron nitride.

United Kingdom Patent Specification 777,000 reports that hot pressedboron nitride articles having an apparent density in excess of 1.9grams/cc. can be produced by hot pressing boron nitride particles whichhad previously been heated in an ammonia atmosphere to reduce the boronoxide content (B 0 Boron nitride articles produced in this manner arereported as having a free boron oxide (B 0 content of as low as 1.67percent. Attempts by the patentees to lower the boron oxide (B 0 contentof such hot pressed articles by heating to 2,100 C. without theapplication of pressure resulted in rupturing of the bodies. Thisrupture was attributed by the patentees to the failure to confine thebody in a mold while heating. This experiment demonstrates that boronnitride articles hot pressed from boron nitride which has been treatedto reduce its boron oxide (B 0 content prior to hot pressing stillrupture upon heating to high temperatures and are unsuitable for hightemperature uses.

Pyrolytic boron nitride having a density in excess of 1.90 grams/cc. hasbeen produced by the reaction of boron trichloride and nitrogen.However, this method has been unsuccessful in producing articles greaterthan V4 inch to #6 inch in thickness (smallest dimension.) Furthermore,the properties of this material are extremely anisotropic in nature.Thus, for example, this material has a flexural strength of about 15,000psi. at room temperature perpendicular to the direction of the layerplanes but below 2500 psi. parallel to the layer planes. The coefficientof thermal expansion parallel to the direction of the layer planes isabout l X 10'/ C. to 1,800 C. compared to about 25 X l0/ C. to 1,800 C.perpendicular to the layer planes.

SUMMARY OF THE INVENTION In accordance with the instant invention it hasnow been discovered that very high purity boron nitride articles havinga high density and a unique acicular crystal structure can be obtainedby treating hot pressed boron nitride articles with a suitable solventto lower their boron oxide (B 0 content and then sintering the treatedmaterial in an inert atmosphere at a temperature of from 1800C. to 2,100C. in the absence of pressure or mechanical restraint. Because of themanner in which these articles are prepared, it is for the first timepossible to prepare large structural shapes of boron nitride e.g.,having a thickness [smallest dimension] in excess of 1 inch) having avery low boron oxide (B 0 content and high density. These shapes arecharacterized by a unique combination of properties not hitherto foundin any available form of boron nitride including excellent hot strength,low and substantially isotropic coefficients of thermal expansion withno irreversible thermal expansion, excellent thermal shock resistance,moisture insensitivity and improved dielectric properties.

The improved boron nitride articles of the instant invention have anoxygen content of less than 0.5 percent by weight and a density of atleast 1.9 grams/cc, generally from 1.9 grams/cc. to 2.1 grams/cc. Whileconventional hot pressed boron nitride is characterized by a decrease inflexural strength as it is heated to temperatures up to about 900 C.,the hot pressed boron nitride of the instant invention is characterizedby an uninterrupted increase in flexural strength with increasingtemperatures up to about 2,000 C. The decrease in flexural strength ofconventional hot pressed boron nitride up to temperatures of about 900C. is caused by the presence of relatively large amounts of boron oxide(B 0 which melts at 525C. and weakens the bonds of the hot pressedarticle causing a sharp drop in flexural strength of from up to about15,000 psi. at room temperature to about 3000 psi. or less at 900C. Coldpressed boron nitride, on the other hand, has a lower boron oxide (B 0content and its flexural strength remains constant at about 3000 psi.when heated from room temperature to 1,000 C. The low flexural strengthof this material is attributed to its low density. By contrast, hotpressed boron nitride treated according to the instant inventionincreases in flexural strength from about 6,000 psi. at room temperatureto as high as 17,000 psi. or more at 2,000 C. At 1,000C., the flexuralstrength of this material is greater than about 6000 psi. vs. about3,000 psi. or less for both conventional hot pressed and cold pressedboron nitride. At 2,000 C., the flexural strength of the treated hotpressed boron nitride increases to as high as 17,000 psi. or more vs.about 8,000 for the untreated hot pressed boron nitride.

Because of the elimination of boron oxide (B the treated hot pressedboron nitride articles of the instant invention undergo no measurableirreversible thermal expansion when heated to l,800C. and subsequentlycooled to room temperature. By way of contrast, conventional hot pressedboron nitride articles undergo a permanent expansion of close to 1percent or more under these conditions. The absence of permanentexpansion of the hot pressed boron nitride articles of the instantinvention and their low thermal expansion characteristics (coefficientof thermal expansion: parallel to the direction of hot pressing, below 2X l0"/C. to 1800C.; perpendicular to the direction of hot pressing,below 3 X C. to l,800C. vs. greater than 4 X lO/C. to 1,800C. and 8 Xl0/C. to 1,800C., respectively, for conventional hot pressed boronnitride) makes these materials extremely resistant to thennal shock andsuitable for use at very high temperatures. While pyrolytic boronnitride also exhibits no permanent expansion after heating to l,800C.and a coefficient of thermal expansion in the direction parallel to thelayer planes of about 1 X l0 /C. to l,800C., this material is extremelyanisotropic in character and its coefficient of thermal expansion in thedirection perpendicular to the layer planes is about 25 X l0 C. to l,800C.

The elimination of boron oxide (B 0 from the treated boron nitridearticles of the instant invention renders these materials extremelyinsensitive to moisture. As a result they pick up less than 1 percent byweight moisture when exposed to 100 percent relative humidity for 100hours at room temperature. Thus, after exposure to the aforesaidconditions, the moisture content of such articles has been measured asabout 0.8 0.9 percent by weight. After standing for 72 hours at roomtemperature at 37 to 45 percent relative humidity, the moisture contentdrops to about 0.2 percent by weight. Conventional cold pressed boronnitride exhibits similar moisture insensitivity, but when conventionalhot pressed boron nitride is treated under the same conditions, it picksup in excess of 2 percent by weight moisture at 100 percent relativehumidity and still possesses at least about 0.5 percent by weightmoisture content after standing for 72 hours at room temperature at 37to 45 percent relative humidity. While the boron nitride of the instantinvention completely loses absorbed water on heating, the moistureabsorbed by conventional hot pressed boron nitride chemically combineswith the boron oxide (B 0 present in this material to form boric acidwhich on heating causes spalling of the hot pressed boron nitride by therapid explusion of water at critical temperatures up to about 300 C.

The improved dielectric properties of the boron nitride articles of theinstant invention are also attributed to the low boron oxide (B 0content of these materials. Typically these materials are characterizedby a loss tangent of less than 0.0006-up to 1,000C. and less than 0.005up to l,400C. as compared to, for example, loss tangents of more than0.003 at 1,000C. and more than 0.010 at 1,400 C. for conventional hotpressed boron nitride at 4.44 to 4.53 61-12 (10 cycles/second). At thelatter temperature, the improved boron nitride articles of the instantinvention exhibit an increase of less than 5 percent in dielectricconstant (4.24 at 1470C. vs. 4.08 at 25 C.). Further, because of themoisture insensitivity of the boron nitride articles of the instantinvention, the electronic properties of these materials aresubstantially unaffected after long exposure to the atmosphere. On theother hand, moisture absorption greatly interferes with the dielectricproperties of conventional hot pressed boron nitride causingunpredictable variations in loss tangent and reducing the attractivenessof this material for electronic applications, particularly at hightemperatures. Dielectric properties of a specimen of boron nitrideprepared in accordance with the instant invention are set forth in TableI below.

TABLE I Dielectric Properties The high purity hot pressed boron nitrideof the instant invention like conventional boron nitride, whether hotpressed, cold pressed or pyrolytic, can be widely utilized because ofits excellent combination of properties such as high thermalconductivity, chemical resistance, high electrical insulation,machinability, nontoxicity and inability to be wet by many molten metalsand halide salts. In addition, this novel material can be prepared instructural shapes of a size not possible from cold pressed or pyrolyticboron nitride. Furthermore, because of its low dissipation factor over awide temperature range this material is well suited for use as microwaveand radar dielectric components (radar windows) whereas high lossfactors present a serious problem when conventional hot pressed boronnitride is used as the dielectric. In addition, its increased moistureinsensitivity, high temperature stability and excellent thermal shockresistance render it useful as plasma arc insulatron.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a photomicrograph having amagnification factor of 500 of the surface of a sample of boron nitridewhich had been hot pressed at a temperature of l,800C. under a pressureof 2,000 psi. and then soaked in methanol to remove boron oxide (B 0 anddried. The view is parallel to the direction of hot pressing.

FIG. 2 is a photomicrograph having a magnification factor of 500 of thesurface of a sample of boron nitride which had been hot pressed at atemperature of 1,800C. under a pressure of 2,000 psi., soaked inmethanol to remove boron oxide (B 0 dried, and then sintered at 2,000C.by the process of the instant invention. The view, as in FIG. 1, isparallel to the direction of hot pressing. The crystal structure of thesample is acicular compared to the more plate-like structure of P16. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The high-purity high densityboron nitride articles of the instant invention are produced by firsttreating conventional hot pressed boron nitride articles to lower theirboron oxide (B 0 content, and then sintering the treated material in aninert atmosphere at a temperature of from l,800C. to 2, C. in theabsence of pressure or mechanical restraint. In order to obtain boronnitride articles having the unique properties described it is necessaryto substantially completely remove boron oxide (B 0 from theconventional hot pressed boron nitride article employed beforesintering.

Substantially complete removal of boron oxide (B from the conventionalhot pressed boron nitride articles employed is effected according to theprocess of the instant invention by immersing the hot pressed materialin a solvent capable of dis- 2,050C. are preferred as the highestflexural strengths are ob" tained at such temperatures. The sinteringtemperature should not be permitted to exceed 2,l00C. as decompositionof the boron nitride occurs above such temperature. Where lower solvingor reacting with boron oxide (B 0 to form soluble flexural strengths aretolerable, e.g., about 3,500 psi. at room reaction products until itundergoes no further weight loss. temperature (perpendicular to pressingdirection), sintering The time required to effect removal of boron oxide(B 0 will temperatures as low as l,600C. can be employed. Sinteringdepend upon the size of the boron nitride piece and the parshould becontinued for about at least 1 hour, preferably at ticular solventemployed. When methanol is employed as 01- l 0 least 2 hours, to obtainthe highest flexural strengths. vent, soaking for 4 days has been foundnecessary for boron The following example is set forth for purposes ofillustranitride cylinders one-half inch in diameter, 18 da s f r li tionso that those skilled in the art may better understand this (1818 1 n hn m er, and 44 days for cylinders 2% in he i invention and it should beunderstood that it is not to be condiameter. With agitation of themethanol solvent, these times stl'ued as hmhihg invention in any mannercan be lowered to 1 day for boron nitride cylinders one-half 5 EXAMPLE 1inch in diameter, 4 days for cylinders 1 inch in diameter, and days forcylinders 2 /inches in diameter. A cylindrical boron nitride plug 14inches long and 14 Among the solvents which can be employed to efi'ectinches in diameter which had been hot pressed at a temperaremoval ofboron oxide (B 0 are water and alcohols, includture of 1,800C. under apressure of 2,000 psi. was machined ing primary, secondary and tertiaryalcohols. The most 20 to produce a number of smaller boron nitridepieces, including preferred alcohols are those alcohols containing up toabout one 3 inch X 2 inch 0.5 inch in size. This piece was dried by fourcarbon atoms. Illustrative of the alcohols which can be heating at 200C,weighed, and then immersed in flowing employed are aliphatic alcoholssuch as methanol, ethanol, water until no further loss in weightoccurred. A period of 14 propanol, isopropanol, butanol, isobutanol,heptanol, octanol, days was required to being the boron nitride piece toconstant glycerol, and the like, and aromatic alcohols such as benzylalweight. During this time the piece was periodically removed cohol,phenethyl alcohol and the like. When water is emfrom the water, dried at200C. and weighed to determine if ployed as solvent, the boron nitridepiece should not exceed any further loss in weight had occurred. Theloss in weight was one-half inch in diameter in order to effect acomplete due to the removal of boron oxide (B 0 and amounted to removalof boron oxide (B 0 The addition of a small 5.2 percent of the originalweight of the piece. The flexural amount of a strong acid such ashydrochloric acid to the water strength of the piece after being broughtto constant weight helps speed up the dissolution of the boron oxide (B0 was 950 psi. at room temperature (perpendicular to pressing Afterimmersion in a suitable solvent, the hot pressed boron direction).nitride is first dried by heating and then further heated in the Afterthe boron nitride piece had been brought to constant absence of pressureor mechanical restraint in an inert atweight, it was heated to 2000C.underanatmosphere of flowmosphere at a sintering temperature in therange of from ing argon over a period of 4% hours (3 hours to heat from1,800C. to 2,100C. By an inert atmo h e i e t an room temperature to1500C., 1% hours to heat from 1,500C. mosphere which is nonreactive withboron nitride under the to and maintained at this temperature for 2heating conditions employed. lnert gases such as nitrogen, [ions] hours-At the end of this the P was weighed helium, neon, argon, krypton,xenon, ammonia, and the like, again and found to have undergone afurther weight loss of provide suitable atmospheres in which hot pressedboron P nitride may be sintered. Temperatures of from 1,950C. to Acylindrical Piece Of boron nitride 6 n h ong nd 2% TABLE 11 BNhotpressed BN hot-pressed at 1,800" O. and at 1,800 O. and

EN containing BNeold-pressed at 2,000 p.s.i.,treat- 2,000 p.s.i.,treat-BN hot-pressed BN hot-pressed 1.65% oxygen hot- 30,000 p.s.i. and Pyroedin water and ed in methanol at 1,800 C. and at 2,000 O. and pressed atsmtered at lytic sintered at 2,000 and sintered at 2,000 p.s.i. 2,000p.s.i. 1,800 C. and 2,000 O. BN

0. 2,000 C. 2,000 p,s.i. Oxygen content (wt. percent) 0.3 0. 3 2. 9 3.01.65 0.3 0 Density, grams/cc 1, 9 2.0 2. 1 2.1 2. 0 1. 6 2Fl9Xl1r1lStr8Ilgl3h, p.s.i.:

7 l l n .0 l 4 are (B):

14, 300 10, 000 3, 272 15, 000 2, 500 3, 000 3, 000 17,000 v 1 V 10, 0002,0000 14, 179 17,000 8, 000 37,000 Coefficient of thermal expan X 10W(room temp. to 1,800 0.):

(A) 0. 72 6.5 4.4 1.7 1.0 (B) 1.95 9.15 3.3 4.4 1.39 25.0Pertcentrslpermane 8 D C a ter eatin tel 00 0 O 1.0 0.7 0.02 0.05 0 00 1. 5 l. 4 0. 45 0 0s 0. to% relative humidity for 100 hrs. at room temwt. percent 0.85 0. 0 3.15 48 t 40 0. 4' 0 Moisture content afterexposure to 100% relative humidity for 100 hr; atlroom fiempaandf 37-45re ative umi it or 72hrs'. \vt. percent 0. 2 o. s "2. 0s 00 0. 21 0 (A)=Measured parallel to pressing direction or the direction of the layerplanes in the (use 01' pyrolytic BN. (B) =Measured perpendicular topressing direction or across the layer planes in the case of pyrolytieEN.

hours exposure.

"140 hours at 100% relative humidity and 16 hours :1t3745% relativehumidity.

* liree cannot he rmule in sull'u-irnt. thickness to measure.

inches in diameter which had been hot pressed at a temperature of 1800C.under a pressure of 2000 psi was sintered as described above afterhaving been immersed in methanol for 44 days to bring it to constantweight. The loss in weight in methanol amounted to 3.3 percent while theloss in weight during sintering amounted to 0.47 percent. The flexuralstrength of the piece after being brought to constant weight in methanolbut before sintering was 980 psi. at room temperature (perpendicular topressing direction).

The properties of the two sintered boron nitride pieces are set forth inTable II below and compared to the properties of a like hot pressedboron nitride piece which had not been treated with water or alcohol andsintered at 2000C., an untreated piece hot-pressed at 2000C. and 2000psi., at boron nitride piece hot pressed at 1800C. and 2000 psi. fromboron nitride containing 1.65 percent by weight oxygen, a boron nitridepiece cold pressed at 30,000 psi. and sintered at 2 000C., and a pieceof pyrolytic boron nitride.

When a boron nitride piece about 0.3 inch thick which had been hotpressed at l,800C. and 2000 psi. was exposed to atmospheric conditionsfor a week and heated to white heat as rapidly as possible with anacetylene torch (heating time to white heat approximately 30 seconds),the surface of the piece exploded into powder and small chunks. Theexplosion was caused by the expulsion of water resulting from thedecomposition of boric acid present in the boron nitride.

When a similar piece of hot pressed boron nitride which had been treatedin water and sintered in accordance with the process of the instantinvention was treated in a similar manner by heating three times towhite heat and quenching twice in cold water no deterioration of thepiece occurred.

When a cylindrical piece of boron nitride inches long and 3% inches indiameter which had been cold pressed under a pressure of 20,000 psi. washeated to 2000C. in a mold without the application of pressure under anatmosphere of flowing argon over a period of 5 hours (400C./hour) andthen maintained at this temperature of 2 additional hours, the pressureexerted by the escape of boron oxide (B 0 during heating caused the moldto crack. Upon cooling, the boron nitride piece was removed fromvthemold and reheated in a furnace to 2000C. as previously described. Theproduct so produced was found to have a density of 1.6 grams/cc, aflexural strength of 2600 psi. at room temperature parallel to thepressing direction, and a flexural strength of 2500 psi. at roomtemperature perpendicular to the pressing direction.

What is claimed is:

1. A process for producing boron nitride articles of manufacture havingan oxygen content of less than 0.5 percent by weight and a densityof atleast 1.9 grams/cc. which comprises treating hot pressed boron nitridewith a solvent capable of removing boron oxide therefrom until said hotpressed boron nitride undergoes no further weight loss, drying the hotpressed boron nitride by heating, and sintering the dried treated boronnitride in an inert atmosphere at a temperature of from 1,600 C. to 2, 100 C. in the absence of pressure.

2. A process as in claim 1 wherein the solvent is selected from thegroup consisting of water and alcohols.

3. A process as in claim 1 wherein the solvent is methanol.

4. A process as in claim 1 wherein the solvent is water and thethickness of the hot pressed boron nitride does not exceed one-halfinch.

5. A process as in claim 1 wherein the sintering temperature is from l,800" C. to 2,100" C.

6. A process as in claim 5 wherein the solvent is selected from thegroup consisting of water and alcohols.

7. A process as in claim 5 wherein the solvent is methanol.

8. A process as in claim 5 wherein the solvent is water and thethickness of the hot pressed boron nitride does not exceed one-halfinch.

9. A process as in claim 1 wherein the sintering temperature is froml,950 C. to 2,050 C.

10. A process as in claim 9 wherein the solvent is selected from thegroup consisting of water and alcohols.

1 l. A process as in claim 9 wherein the solvent is methanol.

12. A process as in claim 9 wherein the solvent is water and thethickness of the hot pressed boron nitride does not exceed one-halfinch.

2. A process as in claim 1 wherein the solvent is selected from thegroup consisting of water and alcohols.
 3. A process as in claim 1wherein the solvent is methanol.
 4. A process as in claim 1 wherein thesolvent is water and the thickness of the hot pressed boron nitride doesnot exceed one-half inch.
 5. A process as in claim 1 wherein thesintering temperature is from 1,800* C. to 2,100* C.
 6. A process as inclaim 5 wherein the solvent is selected from the group consisting ofwater and alcohols.
 7. A process as in claim 5 wherein the solvent ismethanol.
 8. A process as in claim 5 wherein the solvent is water andthe thickness of the hot pressed boron niTride does not exceed one-halfinch.
 9. A process as in claim 1 wherein the sintering temperature isfrom 1,950* C. to 2,050* C.
 10. A process as in claim 9 wherein thesolvent is selected from the group consisting of water and alcohols. 11.A process as in claim 9 wherein the solvent is methanol.
 12. A processas in claim 9 wherein the solvent is water and the thickness of the hotpressed boron nitride does not exceed one-half inch.